There are various types (generally, called “display mode”) of the liquid crystal display device. A display mode in which a liquid crystal cell is arranged between a pair of polarizers disposed in Cross Nicol arrangement is the most common mode. Such a liquid crystal display device provides black display by disposing a pair of polarizers substantially in Cross-Nicol state (dark state) in the following system. Liquid crystal molecules are aligned substantially vertically to substrates such that the liquid crystal cell has a retardation, or alternatively, the liquid crystal cell has a retardation, but an optic axis azimuth of the cell is substantially parallel to or substantially vertical to a polarization axis azimuth (transmission axis azimuth or absorption axis azimuth) of the polarizers by rotating the liquid crystal molecules in the plane, and thereby the retardation is canceled. Such a system is effective in order to enhance contrast. Accordingly, this system has been applied to liquid crystal display devices in various display modes such as Vertical Alignment (VA) mode and In-plane Switching (IPS) mode.
FIGS. 4A to 7A are views each showing viewing angle characteristics of a cross transmittance in four different polarization control systems (i) to (iv). As mentioned below, the polarization control system (i) includes two O-type polarizers; the polarization control system (ii) further includes a retardation film in addition to the polarization control system (i); the polarization control system (iii) includes two E-type polarizers; and the polarization control system (iv) includes one O-type polarizer and one E-type polarizer. FIG. 4A is a view showing azimuth angle dependency of a cross transmittance at a polar angle of 60° in the respective polarization control systems. FIGS. 5A to 7A are views each showing polar angle dependency of a cross transmittance at azimuth angles of 0°, 45°, and 90° in the respective polarization control systems.
As a polarizer commonly used in the liquid crystal display device, for example, a polarizer prepared by uniaxially stretching a polyvinyl alcohol film to which iodine complex and the like has been adsorbed and aligning the iodine complex (hereinafter, also referred to as “conventional iodine polarizer”) is known. If black display is provided using these two conventional iodine polarizers, excellent black display can be obtained in the front direction (at a polar angle of)0°, but light leakage is generated in oblique directions, as shown by (i) in FIG. 4A to 7A. This is because, as mentioned below, the conventional iodine polarizer is a so-called O-type polarizer, and therefore if a transmission axis azimuth of a back surface side-polarizer is not parallel to an absorption axis azimuth of an observation surface side-polarizer in oblique directions (the Cross-Nicol state can not be provided).
In order to solve this problem, a retardation film is conventionally used. That is, the retardation film performs compensation such that also in oblique directions, the transmission axis azimuth of the back surface side-polarizer is apparent parallel to the absorption axis azimuth of the observation surface aide-polarizer (in some display modes, the liquid crystal cell has a retardation in oblique directions, and in such a case, the liquid crystal cell is used as a retardation film). The transmission axis and the absorption axis are fixed in each polarizer, and therefore it is physically impossible to rotate azimuths of these axes. Accordingly, in practice, linearly polarized light (linearly polarized light whose electric field vector oscillation direction (oscillation surface) is parallel to the transmission axis azimuth of the back surface side-polarizer) which has been outputted from the back surface-side polarizer is rotated just by the oscillating direction while its ellipticity is maintained, thereby being converted into linearly polarized light whose electric field vector oscillation direction is parallel to the absorption axis azimuth of the observation surface side-polarizer.
However, a retardation value needed for such conversion varies depending on an observation azimuth and viewing angle (an angle made by line of sight and the normal direction of a screen in the liquid crystal display device, at an intersection of line of sight and the screen of the display device, i.e., an observation direction (angle)). Accordingly, according to the method involving use of the retardation film, as shown by (ii) in FIGS. 4A to 7A, light leakage can be reduced at specific azimuth and viewing angle, but not at every azimuth and viewing angle. In such a point, the method has room for improvement.
For this problem, a method in which a so-called E-type polarizer is used instead of the O-type polarizer, and a method in which an E-type polarizer is used as only one of a pair of polarizers have been proposed (for example, refer to Patent Documents 1 to 3).
Optical characteristics of the O-type polarizer and the E-type polarizer are mentioned below.
According to the conventional iodine polarizer, the stretching direction is an absorption axis and the direction perpendicular to the stretching direction is a transmission axis. Thus, only the absorption axis and the transmission axis of the polarizer in the polarizer plane are often considered. However, it is hardly known that the following point is important, if polarization characteristics of the polarizer, shown for incident light from a direction (oblique direction) other than the normal direction of the polarizer, are considered. That is, it is important which the absorption axis or the transmission axis the polarization axis of the polarizer in the normal direction is, i.e., whether a incident light component whose electric field vector oscillation direction is parallel to the normal direction of the polarizer transmits the polarizer or is absorbed by the polarizer.
The conventional iodine polarizer has a transmission axis in the normal direction of the polarizer, as shown in FIG. 8A. That is, the conventional iodine polarizer satisfies a relationship of Ka>>Kt≈Kz where an imaginary part (also referred to as an “extinction coefficient”) of a complex refractive index relative to light oscillating in the absorption axis azimuth in the polarizer plane is defines as Ka, an imaginary part of a complex refractive index relative to light oscillating in the transmission axis azimuth in the polarizer plane is defined as Kt, and an imaginary part of a complex refractive index relative to light oscillating in the normal direction is defined as Kz. This type of polarizer is a so-called O-type polarizer.
A polarizer shown in FIG. 8B is one whose absorption axis is in the normal direction of the element, that is, a polarizer satisfying a relationship of Kz≈Ka>>Kt. This type of polarizer is a so-called E-type polarizer (for example, refer to Patent Documents 1 and 2). The H-type polarizer is known to provide the Cross-Nicol state at a wider azimuth and viewing angle than those in the O-type polarizer, as shown by (iii) in FIGS. 4A to 7A. If the E-type polarizer and the O-type polarizer are used in combination, as shown by (iv) in FIGS. 4A to 7A, the Cross-Nicol state can be provided at a much wider azimuth and viewing angle (for example, refer to Patent Document 3).
The reason why the E-type polarizer can provide the Cross-Nicol state at a wider azimuth and viewing angle than those in the O-type polarizer is mentioned below. If light (electric field of an optical frequency) enters the polarizer from an oblique direction, the light has a component whose electric field vector oscillation direction is parallel to the normal direction of the polarizer. The E-type polarizer has an absorption axis in the normal direction, and therefore it can absorb the component whose electric field vector oscillation direction is parallel to the normal direction of the polarizer if the light enters the E-type polarizer from an oblique direction. In contrast, the O-type polarizer has a transmission axis in the normal direction of the polarizer, and therefore it transmits the component without absorbing it.
Further, the reason why the combination use of the E-type polarizer and the O-type polarizer can provide the Cross-Nicol state at a much wider azimuth and viewing angle is mentioned below. If the same type of two polarizers, i.e., the E-type of two polarizers, or the O-type of two polarizers, are used for providing the Cross-Nicol state, an transmission axis azimuth of one polarizer is not geometrically parallel to an absorption axis azimuth of the other polarizer when an observation point is in an oblique direction at an azimuth other than the polarization axis azimuth. As a result, light leakage is generated. In contrast, if the E-type polarizer and the O-type polarizer are used in combination, the transmission axis azimuth of the E-type polarizer is parallel to the absorption axis azimuth of the O-type polarizer when an observation point is in the oblique direction. Therefore, light leakage is not generated.
This is mentioned below in more detail with reference to FIGS. 9A, 9B, 10A, and 10B.
FIG. 9A is a planar view schematically showing an arrangement relationship among respective polarization axes when an observation point is in the normal direction of polarizers, in the case that both of a polarizer (back surface side-polarizer) and an analyzer (observation surface side-polarizer) are O-type polarizers and disposed in Cross-Nicol arrangement. FIG. 9B is a plane view schematically showing an arrangement relationship among the respective polarization axes in the configuration shown in FIG. 9A, when an observation point is at a viewing angle tilted at an azimuth of 45° (an azimuth at 45° relative to both of the absorption axis azimuth and the transmission axis azimuth). In this case, as shown in FIG. 9A, the Cross-Nicol state is provided in the normal direction of the polarizer (a transmission axis azimuth 11 of the polarizer is parallel to an absorption axis azimuth 13 of the analyzer). However, as shown in FIG. 9B, the Cross-Nicol state is not provided in an oblique direction (the transmission axis azimuth 11 of the polarizer is not parallel to the absorption axis azimuth 13 of the analyzer). Accordingly, it is found that in the oblique direction, linearly polarized light which has passed through the polarizer is not absorbed by the analyzer.
FIG. 10A is a planar view schematically showing an arrangement relationship of respective polarization axes when an observation point is in the normal direction of polarizers, in the case that the E-type polarizer is used as the polarizer and the O-type polarizer is used as the analyzer. FIG. 10B is a planar view schematically showing an arrangement relationship among the respective polarization axes in the configuration shown in FIG. 10A, when an observation point is at a viewing angle tilted at an azimuth of 45°. In this case, the Cross-Nicol state is provided not only in the normal direction of the polarizers, as shown in FIG. 10A, but also in the oblique direction, as shown in FIG. 10B (the transmission axis azimuth 11 of the polarizer is parallel to the absorption axis azimuth 13 of the analyzer). Although not shown in drawings, the Cross-Nicol state is maintained in the case that an observation point is at a viewing angle tilted at an azimuth other than the azimuth of 45°. That is, if the E-type polarizer and the O-type polarizer are used in combination, the Cross-Nicol state can be provided at every azimuth and viewing angle, in principle. As mentioned above, the E-type polarizer and the O-type polarizer are used in combination, the Cross-Nicol state can be provided at a wider azimuth and viewing angle.
[Patent Document 1]
    Japanese Kohyo Publication No. 2001-504238[Patent Document 2]    Japanese Kokai Publication No. 2001-242320[Patent Document 3]    Japanese Kohyo Publication No. 2003-532141